A fluorophosphate glass is a glass in high demand because of its properties such as low dispersibility, anomalous dispersibility, and high light transmittance over a wide visible range. The low dispersibility and the anomalous dispersibility are effective for correction of chromatic aberration and the high light transmittance is effective as an optical element material for guiding light having a short wavelength, such as near-ultraviolet light, as well as an optical element material for an imaging optical system. In addition, the fluorophosphate glass can provide a filter function to cut near-ultraviolet light by containing copper ions therein and is thus effective as a material for a color compensating filter of a semiconductor imaging device. Such a fluorophosphate glass is disclosed in JP-A-10-139454.
Although such a fluorophosphate glass is a useful optical material, since it shows high volatility in a high temperature state, striae tends to occur when a glass molded article is made from molten glass and therefore it is hard to stably produce high quality glass. In addition, since a volatile ingredient disappear from the molten glass with lapse of time, there also arises a problem that optical characteristics such as a refractive index are easily changed.
To overcome the above problems, JP-A-2007-76958 discloses a method of obtaining glass including sufficiently volatilizing volatile substance from molten glass and then rapidly cooling the molten glass at the point of time when the volatility of the substance is reduced.
This method achieves its required object by putting a difference in refractive index between glasses before and after re-melting in a predetermined range.
Although the invention disclosed in JP-A-2007-76958 provides an excellent technique to overcome the problem inherent in the fluorophosphate glass, there is a need for further improvement from the following standpoints.
In melting of the fluorophosphate glass, a platinum crucible having high corrosion resistance is used to reduce melting of a material of a melting container into glass. However, even when platinum is used, the platinum crucible is slightly eroded by the molten glass and platinum ions are melted and introduced into the glass. Although the temperature of glass is high and accordingly platinum ions are melted and introduced into the glass in a melting process for raw materials or a clarifying process for glass, when the temperature of the glass is decreased to a range of temperature suitable for outflow, the melted and introduced platinum ions are precipitated as particles. Since the platinum ions in the fluorophosphate glass have low solubility, the platinum particles are particularly apt to be precipitated. The precipitated platinum particles are foreign substances acting as a source of scattering of light permeating glass and are the main cause for deterioration of performance of optical elements and glass damage occurring when the glass is irradiated with strong light.
In order to solve such a problem, it is desired not only reducing volatility but also suppressing erosiveness of glass.